The invention also relates to, but is not limited to, use in radar warning systems where a multiplicity of antennas are commonly grouped in a geographic distribution around an aircraft or other vessel to provide direction finding information by comparison of the amplitudes of the strongest to the next strongest received signal. Since a spiral type antenna can provide approximately 90.degree. of spatial coverage, four such antennas are usually deployed at cardinal angle (90.degree.) separations to cover 360.degree. of azimuth simultaneously. In other systems, one or more spirals can be rotated mechanically or used to illuminate a passive fixed or rotating reflector to provide either signal detection from a single direction if fixed, or 360.degree. of detection if scanned. In these and other types of applications, the bandwidth coverage of the antenna is extremely important. Modern technology can provide flat planar cavity backed spiral antennas capable of operating from 0.5 GHz to 22 GHz. The American Electronic Laboratories (AEL) Model AST 1500A is an example of such a device.
In the use of antennas of the type described above, a difficulty arises when millimeter wave reception, defined here as reception of signals greater than 20 GHz, is desired. It becomes necessary to feed the radio frequency (RF) signal from the antenna to the detecting or mixing means by the use of a waveguide type transmission line. It is necessary to minimize losses and to overcome propagation mode effects that exist in coaxial lines when operated at frequencies too high to permit realizable dimensions for the construction of the coaxial line. The use of coaxial lines at the frequencies of millimeter waves can add substantial signal loss as well.
An inherent bandwidth limitation also exists in the use of waveguides, since waveguides exhibit very high loss at frequencies approaching the low frequency cut-off (.lambda..sub.c) of the guide and high attenuation and are subject to spurious mode effects of the dominant mode of propagation (TE.sub.10 mode) at about 1.5 times the low frequency limitation (see Reference Data for Radio Engineers, Fifth Edition, Published by IT&T and H. W. Sams, page 23-8 and following). Typically in going from 18 GHz to 90 GHz, it will be necessary to use four types of waveguides, namely WR-42 from 18 to 26.5 GHz, WR-28 from 26.5 to 40 GHz, WR-19 from 40 to 60 GHz and WR-12 from 60 to 90 GHz, where the WR numbers refer to Electronic Industries Waveguide Designations. Therefore, even if an antenna could be designed to cover a wide range from 18 GHz to 90 GHz for example, the method of transmitting such signals would be extremely complex if not impossible using all of the required waveguides. Although attempts have been made to overcome this problem by the use of ridged waveguides, ridged guides however, have the disadvantage of providing increased attenuation which is counter to the objective of obtaining maximum sensitivity by reducing losses in the transmission system between the antenna and detecting means.
A first solution to this problem has been recognized by Richard P. Flam in his U.S. Pat. No. 4,319,248 entitled Integrated Spiral Antenna-Detector Device, in which a detector diode is placed at the center feed points of a spiral antenna, with the detected signals removed from the antenna at the outer periphery. This embodiment effectively eliminates the transmission line, permitting direct video detection of all signals received by the spiral antenna. The spiral antenna elements are mounted in front of a cavity which is lined with RF anechoic material, for the purpose of completely absorbing the back radiating wave and providing wide-band coverage at the penalty of 3 db of antenna gain. A commercial version of this antenna is offered for sale by American Electronic Laboratories, Inc. under the trademark ANTECTOR (Catalog 7827.5M, page 3a). The prior art devices, however, do not provide the high sensitivity over a broad-band attained by the present invention utilizing superheterodyne or superregenerative detection by an integrated receiver antenna device having a detector mixer unit at the center feed points of a spiral antenna.